The present invention relates to height detection and levelling, for example of the substrate and/or mask, in lithographic apparatus. More particularly, the invention relates to a system for off-axis levelling in lithographic projection apparatus comprising:                a radiation system for supplying a projection beam of radiation;        a first object table provided with a mask holder for holding a mask;        a second, movable object table provided with a substrate holder for holding a substrate;        a projection system for imaging an irradiated portion of the mask onto a target portion of the substrate; and        a positioning system for moving said second object table between an exposure position, at which said projection system can image said mask portion onto said substrate, and a measurement position.        